A new analysis by the March of Dimes and the CDC shows that the two statistics are very much related.

The analysis revealed an increase of nearly 60,000 preterm deliveries among single-birth pregnancies between 1996 and 2004, with more than nine out of 10 of these deliveries performed by C-section.

Single-birth pregnancies increased by about 10% during this period, while C-section deliveries among women giving birth to just one baby increased by 36%.

Late preterm deliveries, defined as births between 34 and 36 weeks of gestation, accounted for the vast majority of the increase in preterm births among this group.

There is concern that at least some of these early deliveries may not be medically warranted, Joann R. Petrini, PhD, MPH, of the March of Dimes, tells WebMD.

Petrini is director of the March of Dimes' Perinatal Data Center.

"We know that more women over 35 are giving birth, and more pregnant women have other risk factors for problem deliveries, including hypertension, obesity, and diabetes," she says. "But studies suggest that these risk factors do not fully explain the increase in C-section deliveries."

Late Preterm Births Troubling

Nor does the widely reported, but hard to pin down, practice of performing cesarean sections solely for the convenience of the mother or the physician.

The practice is widely known as C-section on demand. Ob-gyn professor Sarah Kilpatrick, MD, says it accounts for only a small percentage of surgical deliveries overall.

Kilpatrick heads the division of maternal and fetal medicine at the University of Illinois at Chicago.

"It is true that we are seeing more C-sections on demand than we saw 10 years ago, but the numbers are still pretty small," she tells WebMD.

It is more likely that the increase in C-sections is being driven by liability concerns among physicians, maternal risk factors, and the fact that fewer hospitals are performing vaginal births on women who have had prior C-sections.